This invention relates to an insulating resin composition which is suited for a solder resist or a plating resist in the fabrication of circuit boards and for an insulating film and a photosensitive adhesive for the build-up of multilayer wiring boards for mounting semiconductor devices, to a laminate obtained from said composition and to a cured product of said composition.
Following the trend of electronic instruments towards advanced miniaturization and higher performance in recent years, there is a trend toward higher circuit density in the printed circuit boards which are used in electronic instruments and insulating resin materials to be used in printed circuit boards are required to show finer processibility. Patterning by exposure to light and development has been known as an effective technique for fine processing of insulating resin materials; this technique has relied on the use of photosensitive resin compositions and there are presently a number of performance requirements for them, for example, photosensitivity, adhesiveness to the substrate and reliability represented by impact resistance and resistance to plating.
Some insulating materials for printed wiring boards aim at preventing an occurrence of cracking and letting impact resistance and heat resistance coexist with electrical insulation reliability and a technique of incorporating fine particles of crosslinked elastic polymer with an average diameter of 5 xcexcm or less in an insulating resin composition is disclosed in JP10-147685 A. However, the technique disclosed here yielded dispersed particles still too large in diameter and was unable to realize the coexistence of impact resistance and adhesiveness to the substrate at a sufficiently high level.
On the other hand, a resin composition containing finer particles with a diameter on the order of 70 nm is disclosed in JP8-139457 A. As pointed out later by JP10-182758 A, however, this resin composition cannot provide a sufficiently high peel strength against a plating metal because of insufficient anchor depth and surface roughness when used for insulation. JP10-182758 A discloses a composition formulated by the simultaneous use of a crosslinked elastic polymer with a particle diameter of less than 1 xcexcm and a matter with a particle diameter of 1-10 xcexcm. However, the technique disclosed here requires the crosslinked elastic polymer in a relatively large amount and, in the case where a base resin of high heat resistance is used, the incorporation of the crosslinked elastic polymer in question undesirably deteriorates the heat resistance of the insulating resin layer containing such base resin.
An object of this invention is to provide an insulating resin composition which exhibits excellent adhesiveness to plating metals, heat resistance, resolution characteristics and impact resistance, comprises a crosslinked elastic polymer amenable to selective etching by a permanganate salt that is used in an ordinary plating operation and is suited for an insulating resin layer in a multilayer printed wiring board and to provide a laminate such as dry film formed from said composition.
The insulating resin composition of this invention comprises a crosslinked elastic polymer in a photo-polymerizable or thermally polymerizable resin composition at the rate of 3-10 parts by weight of said crosslinked elastic polymer per 100 parts by weight of a resin component (excluding said crosslinked elastic polymer and including monomers) and said crosslinked elastic polymer has carboxyl groups and is dispersed with an average secondary particle diameter in the range of 0.5-2 xcexcm. This insulating resin composition can be laminated to a separable support. Furthermore, this invention relates to a polymer or a cured product of the aforementioned insulating resin composition.
This invention will be described in detail below.
The insulating resin composition of this invention comprises a crosslinked elastic polymer and a resin component comprising a photo-polymerizable or thermally polymerizable resin or monomer as main ingredients. The resin component here is adequate if it comprises one kind or more of photo-polymerizable or thermally polymerizable resins, oligomers and monomers and is subject to no other restriction. The resin component may be heat-curable or photo-curable or both. Moreover, the resin component may be photosensitive and preferably comprises a carboxyl-containing copolymer which can be developed by an aqueous alkaline solution. Since the resin, oligomer or monomer in the insulating resin composition with the exception of the crosslinked elastic polymer forms a matrix resin phase after polymerization or curing, it is occasionally referred to as resin component or matrix resin component in the following description.
The resin component comprises, as main ingredients, (A) a carboxyl-containing copolymer resulting from the reaction of a diol with a polyvalent carboxylic acid and having a weight average molecular weight of 3,000-40,000 and an acid value of 50-200 mgKOH/g, (B) an unsaturated compound containing one or more photo-polymerizable ethylenic unsaturated linkages in the molecule, (C) an epoxy resin and (D) a photopolymerization initiator and, preferably, the resin component comprises 100 parts by weight of the sum of components (A) and (B), 10-30 parts by weight of component (C) and 0.1-15 parts by weight of component (D).
Diols useful for the preparation of the aforementioned carboxyl-containing copolymer (A) preferably possess a symmetrical molecular structure from the viewpoint of increasing the molecular weight during polymerization because the symmetrical structure renders two hydroxyl groups in a diol molecule equally reactive with two carboxyl groups in a polyvalent carboxylic acid, preferably with two acid anhydride groups in an acid dianhydride.
Concrete examples of diols are ethylene glycol, diethylene glycol, polyethylene glycol, polypropylene glycol, hydrogenated bisphenol A, bis(4-hydroxyphenyl) ketone, bis(4-hydroxyphenyl) sulfone, 2,2-bis(4-hydroxyphenyl)propane, bis(4-hydroxyphenyl) ether, bis(4-hydroxyphenyl)hexafluoropropane, 9,9-bis(4-hydroxyphenyl)fluorene, bis(4-hydroxyphenyl)dimethylsilane and 4,4xe2x80x2-biphenol. Examples also include addition compounds of various diglycidyl ethers of the aforementioned diols and (meth)acrylic acid, addition products of alicyclic epoxy compounds and (meth)acrylic acid and addition products of the aforementioned bisphenols and ethylene oxide or propylene oxide. Particularly preferred from the viewpoint of improved sensitivity to light exposure and enhanced resolution are the addition products of (meth)acrylic acid because they possess polymerizable unsaturated linkages and alkali-soluble carboxyl groups in the same molecule after the reaction with a polyvalent carboxylic acid.
Among carboxyl-containing copolymers, resins containing a fluorene skeleton in their unit structure (hereinafter referred to as fluorene skeleton-containing resin) are desirable for manifestation of excellent heat resistance and the use of the carboxyl-containing copolymer in which the fluorene skeleton-containing resin accounts for 30 wt % or more, preferably 50 wt % or more, is effective for manifesting heat resistance of the insulating resin composition.
Particularly preferred fluorene skeleton-containing resins are those which possess a fluorene skeleton and are obtained by the reaction of a fluorene epoxy(meth)acrylate represented by the following general formula (1) 
(wherein R1 and R2 are hydrogen or methyl group and different from or identical with each other and R3-R10 are hydrogen, an alkyl group with 1-5 carbon atoms or halogen and different from or identical with one another) with a polyvalent carboxylic acid or its acid anhydride. The reaction of a fluorenebisphenol type epoxy(meth)acrylate represented by general formula (1) with a polyvalent carboxylic acid or its acid anhydride yields an alkali-soluble product.
Polyvalent carboxylic acids include their acid anhydrides, acid chlorides and the like and acid anhydrides are preferred. Examples of polyvalent carboxylic acids are maleic acid, succinic acid, itaconic acid, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, chlorendic acid, methylendomethylenetetrahydrophthalic acid, methyltetrahydrophthalic acid, pyromellitic acid, benzophenonetetracarboxylic acid, biphenyltetracarboxylic acid and diphenylethertetracarboxylic acid. Polyvalent carboxylic acids preferably contain, at least partly, tetracarboxylic acids or their dianhydrides. The aforementioned polyvalent carboxylic acids may be used singly or as a mixture of two kinds or more.
The reaction of a diol such as an epoxy(meth)acrylate with a polyvalent carboxylic acid can be effected by a known method. A polycarboxylic acid to be used is preferably the acid anhydride(s) of one or a mixture of tribasic and higher acids in order to control the acid value of the resulting resin at 10 mgKOH/g or more for full manifestation of alkali solubility.
As for a method for the preparation of carboxyl-containing copolymer (A), the one described in JP7-3122 A may be adopted.
In the case where an unsaturated compound (B) containing one or more photo-polymerizable ethylenic unsaturated linkages in the molecule is used, typical examples of such unsaturated compounds are acrylates including hydroxyl-containing (meth)acrylates such as polyethylene glycol (meth)acrylate and butanediol mono(meth)acrylate, aliphatic (meth)acrylates such as allyl (meth)acrylate, butoxytriethylene glycol (meth)acrylate, glycidyl (meth)acrylate, methacryloyloxypropyltrimethoxysilane, tetrafluoropropyl (meth)acrylate and dibromopropyl (meth)acrylate, alicyclic (meth)acrylates such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate, aromatic (meth)acrylates and phosphorus-containing (meth)acrylates.
Moreover, examples include bifunctional compounds such as diethylene glycol di(meth)acrylate, bisphenol A di(meth)acrylate and tetrabromobisphenol A di(meth)acrylate. Still more, examples include trifunctional and higher compounds such as trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol tetra(meth) acrylate, dipentaerythritol hexa(meth)acrylate, alkyl-modified dipentaerythritol penta(meth)acrylate and urethane tri(meth)acrylate.
The products obtained by modifying the aforementioned monofunctional, bifunctional, trifunctional and higher compounds containing ethylenic unsaturation with caprolactone, propylene oxide or ethylene oxide can be used in the same manner. Other polymerizable monomers, for example, monofunctional vinyl compounds such as vinyl acetate, vinylcaprolactam, vinylpyrrolidone and styrene, can also be used in case of need. Polyesters and vinyl polymers can also be used in case of need. The aforementioned compounds of varying functionality and their modified products and resins can be used not only singly but also as a mixture of two kinds or more. The average number of ethylenic unsaturated bonds in one molecule is preferably 1.5 or more.
In the case where excellent photo-curability or higher photosensitivity is required in addition to alkali solubility for the insulating resin composition of this invention, it is preferable to incorporate a resin or monomer having two (bifunctional) or more, preferably three (trifunctional) or more, polymerizable double bonds in the molecule. The amount to be used of unsaturated compound (B) having one or more photo-polymerizable ethylenic unsaturated linkages is preferably in the range of 10-200 parts by weight per 100 parts by weight of carboxyl-containing copolymer (A).
Examples of epoxy resin (C) to be incorporated include epoxy resins such as phenol novolak epoxy resins, cresol novolak epoxy resins, bisphenol A epoxy resins, bisphenol F epoxy resins, bisphenol S epoxy resins, biphenyl epoxy resins and alicyclic epoxy resins and compounds having at least one epoxy group such as phenyl glycidyl ether, p-butylphenyl glycidyl ether, triglycidyl isocyanurate, diglycidyl isocyanurate, allyl glycidyl ether and glycidyl methacrylate.
In the case where the matrix resin is rendered alkali-soluble, the amount of epoxy resin to be used is kept within such a range as to maintain the property of alkali solubility or preferably in the range of 10-30 parts by weight per 100 parts by weight of the sum of the components (A) and (B).
Examples of photopolymerization initiator (D) to be incorporated include radical generators such as Michler""s ketone and cation generators such as triarylphosphonium salts and diaryliodonium salts. Such initiators can be used singly or as a mixture of two kinds or more. The amount of photopolymerization initiator to be used is preferably 0.1-15 parts, more preferably 1-5 parts by weight per 100 parts by weight of the sum of the components (A) and (B). Use in excess of 15 parts by weight increases light absorption and there is the possibility of light not penetrating to the lower part.
The photopolymerization initiator may be incorporated in combination with a known photosensitizer such as ethyl N,N-dimethylaminobenzoate, isoamyl N,N-dimethylaminobenzoate, triethanolamine and triethylamine. Photosensitizers may be used singly or as a mixture of two kinds or more and the amount to be used is preferably 10-70 wt % of the photopolymerization initiator.
The insulating resin composition of this invention is a dispersion of the carboxyl-containing crosslinked elastic polymer in the matrix resin phase. The crosslinked elastic polymer is incorporated in the insulating resin composition of this invention to achieve the following objective. The crosslinked elastic polymer disperses in the insulating resin composition with the secondary particle diameter controlled in the specified range to form a sea-island structure and, after curing, forms the dispersed phase with a particle diameter of 0.5-2 xcexcm in the insulating resin layer thereby providing said insulating resin layer with cracking resistance. At the same time, the cured resin layer is subjected to the dissolving action of a roughening solution to create unevenness on its surface for anchor effect and improved adhesion of the resin layer to the plating metal.
Examples of the crosslinked elastic polymer to be used in this invention include carboxyl-containing crosslinked acrylic rubber, carboxyl-containing crosslinked NBR and carboxyl-containing crosslinked MBS. Carboxyl-containing acrylic rubber and carboxyl-containing NBR are particularly preferred in respect to roughening as they are easy to dissolve and carboxyl-containing acrylic rubber is preferred in respect to insulation properties after curing.
The amount of the carboxyl group of the crosslinked elastic polymer to be incorporated is preferably 1-50 (mgKOH/g) in terms of acid value. An acid value of less than 1 deteriorates adhesiveness to the resin composition and the solubility during development and roughening. On the other hand, an acid value in excess of 50 deteriorates moisture resistance reliability and storage stability of the resin composition.
The primary particle diameter of the crosslinked elastic polymer is 0.1 xcexcm or less, preferably in the range of 0.03-0.9 xcexcm. The secondary particle diameter determined by observing the cured polymer by a transmission electron microscope (TEM) and applying the area average method is in the range of 0.5-2 xcexcm, preferably in the range of 0.6-1.5 xcexcm. Preferably, 80% or more of the particles exists in the aforementioned range.
The amount of the crosslinked elastic polymer to be used needs to be 3-10 parts by weight, preferably 4-8 parts by weight, per 100 parts by weight of the aforementioned matrix resin or resin component. Use of less than 3 parts by weight does not yield sufficient adhesiveness to the plating metal and manifestation of impact resistance becomes difficult. On the other hand, use in excess of 10 parts by weight presents no problem in respect to impact resistance and adhesiveness, but the heat resistance of the resin layer deteriorates and, besides, the crosslinked elastic polymer which dispersed once develops a tendency to agglomerate again; this means that, when the insulating resin composition of this invention is made into a varnish, the crosslinked elastic polymer separates and precipitates and this prevents a stable operation in coating of the varnish.
It is possible to add a flocculated to the insulating resin composition of this invention to stabilize the flocculation of the fine particles of the crosslinked elastic polymer and control the secondary particle diameter. A common polymeric flocculant is useful as a flocculant of this kind and examples include cationic polymers such as sodium alginate, poly(sodium acrylate), salts of partial hydrolyzates of polyacrylamide and maleic acid copolymers, anionic polymers such as water-soluble aniline resins, polythiourea, polyethyleneimine, quaternary ammonium salts and polyvinylpyridines and nonionic polymers such as acrylamide and polyoxyethylene.
Furthermore, it is allowable to incorporate in the insulating resin composition of this invention one or more of inorganic fillers such as silica, alumina, titanium oxide and boron nitride for the purpose of lowering thermal expansion and improving elasticity modulus and moisture absorption of the cured product. Still more, it is allowable to incorporate additives such as curing accelerators of epoxy resins, polymerization inhibitors, plasticizers, leveling agents, anti-foaming agents and colorants as occasion demands. In addition, curing agents of epoxy resins, solvent-soluble resins and the like may be incorporated and the curing agents and the resins are calculated as matrix resin.
Curing accelerators of epoxy resins include amines, imidazoles, carboxylic acids, phenols, quaternary ammonium salts and methylol-containing compounds. Polymerization inhibitors include hydroquinone, hydroquinone monomethyl ether, pyrogallol, tert-butylcatechol and phenothiazine. Plasticizers include dibutyl phthalate, dioctyl phthalate and tricresyl phosphate. Anti-foaming agents and leveling agents include silicones, fluorine-containing compounds and acrylic compounds.
The viscosity of the resin composition for electrical insulation of this invention can be controlled by incorporating a solvent as needed. A solvent of this kind must be capable of dissolving the resin component of the aforementioned photosensitive resin composition and, in addition, inert to the resins in the matrix resin component and to the additives. The solvent to be used must satisfy these conditions, but is subject to no other restriction.
The insulating resin composition of this invention can occur in the following states: (i) a solid or a liquid without a solvent, (ii) a liquid (varnish) formed by dissolving the composition in a solvent, (iii) a dry matter formed by drying the varnish and (iv) any of the foregoing three in which a filler is incorporated. In the case where the insulating resin composition of this invention occurs in any of the states (i) to (iv), the proportions of the matrix resin component and the crosslinked elastic polymer are in the aforementioned range and the crosslinked elastic polymer accounts for 3-10 wt %, preferably 5-9 wt %, of the whole and the matrix resin component accounts for 80-97 wt %, preferably 85-97 wt %, more preferably 90-95 wt %, of the whole. In addition, additives may be incorporated in small amounts.
Moreover, the insulating resin of this invention may occur in the state of (V) a solid resulting from polymerization or curing of the insulating resin composition and its resin phase contains of the dispersed phase made up of the crosslinked elastic polymer and the matrix resin phase made up of the matrix resin component. In the case of state (V), the crosslinked elastic polymer in the cured product accounts for 3-10 wt %, preferably 5-9 wt %, of the whole and the matrix resin phase accounts for 80-97 wt %, preferably 85-97 wt %, more preferably 90-95 wt %, of the whole. In the states (i) to (iv) where solvents and fillers are incorporated, the aforementioned proportions preferably hold under conditions where the solvents and fillers are excluded. The conditions where the solvents and fillers are excluded mean that they are excluded in calculating the proportions. As noted above, the matrix resin component consists of components A-D and other ingredients such as a curing agent of epoxy resin and the like.
The method for preparing the insulating resin composition of this invention will be explained next. The insulating resin composition of this invention can be formulated, for example, from the crosslinked elastic polymer which has been dispersed in a solvent to primary particles with a diameter of 100 nm or less in advance, the aforementioned carboxyl-containing copolymer (A), unsaturated compound (B) containing one or more photo-polymerizable ethylenic unsaturated linkages in the molecule, epoxy resin (C), photopolymerization initiator (D) and other additives such as photo-initiation aids, coupling agents and antioxidants. It is desirable in the formulating step to use a solvent to control the viscosity of the resin composition solution. The resin composition solution containing the crosslinked elastic polymer is agitated in a known agitator at normal temperature for several minutes to several hours to yield a resin composition solution in which the crosslinked elastic polymer is dispersed to an average secondary particle diameter in the range of 0.5-2 xcexcm. The average secondary particle diameter of the crosslinked elastic polymer can be brought into the range of 0.5-2 xcexcm by suitably controlling the kind of agitator and agitating element, agitating speed and shape of vessel. Crosslinked elastic polymers dispersed in advance to primary particles with a diameter of 100 nm or less are commercially available and one of them may be used here.
If agitated not sufficiently in the step for agitation, large agglomerates would form with an average secondary particle diameter of 2 xcexcm or more and manifestation of impact resistance becomes difficult. If agitated too vigorously, the crosslinked elastic polymer would undergo secondary agglomeration with difficulty and disperse close to the primary particle diameter thereby undesirably deteriorating the adhesive strength of the plated metal at the time of roughening. In order to stabilize the secondary particle diameter, the resins are mixed and then the mixture may be put through a filter with a pore diameter of 0.5-5 xcexcm. A filter with a pore diameter of less than 0.5 xcexcm would not only divide secondary agglomeration further but also present increased resistance to filtration to its disadvantage. A filter with a diameter of 5 xcexcm or more would allow singularly large secondary agglomerates to remain in the filtered mixture and undesirably deteriorate the mechanical properties.
The insulating resin composition of this invention can be used, for example, in the following manner: (1) the composition is prepared as a varnish and applied to the object to form a layer of insulating resin and (2) a separable support is coated with the composition and stripped of the solvent to yield a laminate (dry film).
When used as a varnish, the insulating resin composition of this invention is made into a varnish and the substrate is coated with the varnish by such means as spin coating and curtain coating, dried, exposed to light, developed to form a pattern and heat-cured. When used as a pre-formed laminate, the support is coated uniformly with the insulating resin composition of this invention, stripped of the solvent by hot-air drying or the like, covered by a protective film if necessary and wound into a roll. The drying temperature is preferably 80-120xc2x0 C. in consideration of the heat stability of the unsaturated compound and the productivity. Furthermore, the temperature is raised preferably in multiple steps in order to prevent skinning and foaming on the surface of the applied coating during drying.
The organic solvent frequently remains in the resin layer after drying and it is desirable to reduce the content of the residual organic solvent to 15 wt % or less, preferably 10 wt % or less. The content here refers to the loss in wt % when the resin layer after drying whose weight is taken as 100 wt % is heated again at 200xc2x0 C. for 30 minutes to the absolute dry weight. Cold flow tends to occur when the content of the residual organic solvent exceeds 15 wt %.
The thickness of the insulating resin layer after drying varies with the end use and it is 1-10 xcexcm for liquid crystal displays and 5-100 xcexcm for circuit boards. Resolution improves as the resin layer becomes thinner and it becomes possible to form vias and lines with the diameter or width equal to or less than the thickness of the resin layer. For example, when the film thickness is 30 xcexcm, it is possible to form 30 xcexcm vias and 20 xcexcm lines and spaces. Moreover, when the film thickness is 5 xcexcm, it is possible to form a 20 xcexcm isolated line or isolated dot.
The support (film) to which the resin composition is applied is preferably a transparent material which lets energetic radiation pass through. Examples of such a support include publicly known poly(ethylene terephthalate) films, polyacrylonitrile films, polypropylene films for optical use and films of cellulose derivatives. The film thickness is generally 10-30 xcexcm out of necessity to maintain a certain film strength, although thinner films are more advantageous in respect to image formation and economy. In the laminate of this invention, the surface of the insulating resin layer on the side not in contact with the support can be laminated to a protective film as occasion demands. Preferably, the protective film here adheres to the insulating resin layer with a sufficiently lower strength than the support and is readily separable. An example of the protective film of this kind is polyethylene film.
Known techniques can be applied to the fabrication of circuit boards, multi-chip modules and color filters and spacers for liquid crystal displays by the use of the laminate of this invention. The fabrication of a circuit board is taken as an example and the steps involved are described briefly below.
In case the laminate has a protective film, the protective film is peeled off and the insulating resin layer is laminated to the surface of the substrate under heat and pressure by the use of a hot-roll laminator and the like. The heating temperature in this step is 70-120xc2x0 C., preferably 80-110 xc2x0 C. The adhesiveness to the substrate deteriorates below 70xc2x0 C. while the insulating resin layer protrudes from the edges adversely affecting the precision of film thickness above 120xc2x0 C. The support is then peeled off and the insulating resin layer is exposed to energetic radiation through a mask for image transfer. Thereafter, the insulating resin layer is developed by an aqueous alkaline solution and the unexposed portion is removed. Useful here as a developer is an aqueous solution of sodium carbonate, potassium carbonate, potassium hydroxide, diethanolamine or tetramethylammonium hydroxide. A developer suited with the characteristics of the resin layer is selected and it may be used together with a surfactant. Finally, polymerization or curing (occasionally the two are put together and referred to as curing) is completed thermally to yield insulating resin, for example, permanent insulating film. In order to provide the resin with heat resistance or to provide the resin matrix with lower etching quality than that of the crosslinked elastic polymer in the etching step, the heat curing here is effected preferably in the range of 160-200xc2x0 C.
The surface of heat-cured resin layer is planarized by buffing if necessary, then roughened by the publicly known desmear process with the use of a permanganate salt, and submitted to electroless copper plating and, if necessary, to electrolytic copper plating by a known technique to form a conductive layer. Annealing is preferably applied after electrolytic copper plating. A circuit is formed by selective etching of the conductive layer and a multilayer circuit board is formed by repeating the steps starting with the lamination of the insulating layer.
The insulating resin composition of this invention exhibits excellent impact resistance and high adhesiveness to the plating metal due to manifestation of the anchor effect after surface roughening. Moreover, the crosslinked elastic polymer can produce the anticipated effect when added in a relatively small amount and this helps to make the most of the intrinsic characteristics of the matrix resin constituting the resin composition; in case matrix resin of excellent heat resistance is used, the degradation of the heat resistance can be minimized. As noted above, the insulating resin composition of this invention can be used in a variety of applications requiring high impact resistance, good adhesiveness to the plating metal and good heat resistance and are useful as peripheral materials of electronic parts such as semiconductor devices. Moreover, the crosslinked elastic polymer used in this invention can be etched selectively by a permanganate salt which is used in an ordinary plating operation and is, in particular, extremely useful as a material for the insulating layer in multilayer printed circuit boards. Still more, the laminate of this invention not only has the properties of the insulating resin composition but also is easy to handle and yields high productivity when used in the formation of the insulating layer in multilayer wiring boards.